According to the type defined in more detail in the preamble of Claim 1, the invention concerns a method of controlling a CVT automatic transmission in which an electronic transmission control comprises a main pressure and a contact pressure of a secondary pulley.
Modern automatic transmissions work with an electronic transmission control (EGS) by means of which can be met criteria relevant to information an safety. The electronic transmission control which as a rule communicates with an engine control unit from the transmission motor and side of the vehicle. From the input signal and stored data such as gear shift characteristic lines for gears, tuning parameters for pressure calculation, engine engagement and time steps, the same as regulators and diagnosis parameters, the electronic transmission control calculates the correct gear and optimum pressure curves for gear shifts by means of adequate program modules.
To ensure a correct operation of the automatic transmission, great importance is attached to the reliability of the input signals, since otherwise there is chosen a shifting program not adapted to the operation situation which under circumstances can result in damage to the transmission and dangerous situation in the operation.
In CVT automatic transmissions, together with the rotational speeds of the primary pulley and secondary pulley of the variator, the main pressure and the contact pressure of the secondary pulley which is decisive for the variator""s capability of torque transmission, especially constitute important input variables on which the correctness of the transmission control essentially depends.
DE 196 50 218 A1 discloses a method for monitoring the orderly operation of two rotational speed sensors associated with the primary and secondary cone pulley pairs wherein brief failure of the rotation speeds can also be detected and react thereto with adequate flexibility. This is obtained by the fact that in the presence of a stationary state during travel, a ratio of the variator is determined from the rotational speed signals of the primary and secondary pulleys and the ratio is coordinated with a first characteristic field, the characteristic field having a range of admissible and a range of non-admissible ratios. An error is detected when the ratio is in a non-admissible range and with the detection of the error the contact pressure level of the variator is raised in a first step and if the presence of the error persists, an emergency drive program is activated in a second step.
While with the method known already, a monitoring of the signals delivered by the rotational speed sensors is possible, the verification of the signals issued by the pressure sensors to the electronic transmission control is unsatisfactory.
In the practice, two sensors are provided for pressure monitoring, namely, one sensor after a main-pressure supply pump and one sensor on the secondary pulley or hydraulic chamber thereof.
When the sensors or the current regulators thereof are defective, erroneous pressure values are issued. The same applies when a failure occurs in the pressure preparation, e.g. in case of a mechanical defect in the transmission. Even though in that case, the pressure values can lie outside a tolerance range, the sensing of the electronic transmission control is not detected as defect so that an incorrect adaption is effected in that case.
The problem, on which this invention is based, is to make available a method for control of a CVT automatic transmission in which an electronic transmission control comprises a main pressure and a contact pressure of a secondary pulley, the pressure values being verified as to plausibility.
The inventive method offers the advantage that in the sensing of the main pressure and of the contact pressure of the secondary pulley of the variator failures can be reliably detected so that further processing of non-plausible pressure values can be prevented.
Since for reasons of consumption an objective of the control of the CVT automatic transmission consists in keeping the pressure level as high as needed but as low as possible, the inventive method makes it possible to implement a transmission control optimated in terms of consumption and with which is prevented a pressure availability too high on account of an error in the pressure sensing.
Besides, it is possible with the inventive method advantageously to ensure that seriously erroneous pressure values cannot lead to any control of the CVT automatic transmission that can damage the automatic transmission or impair the stability of the vehicle.